Water-soluble cellulose ethers, typically hydroxypropyl methyl cellulose (HPMC), which belong to a class of water-soluble polymers have thickening effects even in hydraulic compositions. They are widely used in concrete applications including anti-washout underwater concrete (for inhibiting segregation during underwater placement), spraying concrete (for preventing dust generation during spraying), and self-consolidating concrete (for inhibiting segregation during placement).
Hydraulic compositions are mixtures of a hydraulic substance such as cement, aggregates such as fine and coarse aggregates, water, a water-reducing agent, and other admixtures. They are generally prepared in a fresh concrete plant. When a hydraulic composition containing a thickener such as a water-soluble cellulose ether as mentioned above is prepared in a ready-mixed concrete plant, the premixed thickener can have detrimental effects to the batch, typically a loss of slump value and an increase of air content. Thus a fresh concrete composition is previously prepared by mixing a hydraulic substance, aggregate, water, water-reducing agent, and other ingredients (excluding thickener) in a fresh concrete plant, and a thickener is added thereto at job site.
However, a problem arises when a powdery thickener is added to a fresh concrete composition. Since the amount of thickener added is very small, it is difficult to add the desired amount of thickener due to powder scattering. The resulting composition fails to exert satisfactory hydraulic effects. For this reason, the thickener is generally added in aqueous solution form. Regrettably, the thickener aqueous solution has a high viscosity, which inhibits addition in high concentration.
To overcome these drawbacks, for example, a water-soluble cellulose ether which has been treated with glyoxal for crosslinking reaction into a temporary water-insolubilized state is added to a fresh concrete composition at job site (see Non-Patent Document 1). With this approach, water-soluble cellulose ethers not having thickened can be added to fresh concrete compositions at sites of placement.